Pluripotent stem cells have the potential to differentiate into the full range of daughter cells having distinctly different morphological, cytological or functional phenotypes unique to a specific tissue. By contrast, descendants of pluripotent cells are restricted progressively in their differentiation potential, with some cells having only one fate. Pluripotent cells have extraordinary scientific and therapeutic potential, as they can be differentiated along the desired differentiation pathway in a precisely controlled manner and used in cell-based therapy.
Two categories of pluripotent stem cells are known to date: embryonic stem cells and embryonic germ cells. Embryonic stem cells are pluripotent stem cells that are derived directly from an embryo. Embryonic germ cells are pluripotent stem cells that are derived directly from the fetal tissue of aborted fetuses. For purposes of simplicity, embryonic stem cells and embryonic germ cells will be collectively referred to as “ES” cells herein.
ES cells are presently obtained via several methods. In a first method, an ES cell line is derived from the inner cell mass of a normal embryo in the blastocyst stage (See U.S. Pat. No. 6,200,806, Thompson, J. A. et al. Science, 282:1145-7, 1998 and Hogan et al., 2003). A second method for creating pluripotent ES cells utilizes the technique of somatic cell nuclear transfer (SCNT). In this technique, the nucleus is removed from a normal egg, thus removing the genetic material. Next, a donor diploid somatic cell is placed next to the enucleated egg and the two cells are fused, or the nucleus is introduced directly into the oocyte by micromanipulation. The fused cell has the potential to develop into a viable embryo, which may then be sacrificed to remove that portion of the embryo containing the stem cell producing inner cell mass.
In a third method, the nucleus of a human cell is transplanted into an entirely enucleated animal oocyte of a species different from the donor cell (referred to herein as animal stem cell nuclear transfer, or “ASCNT”). See U.S. Pat. Application Ser. No. 20010012513 (2001). The resultant chimeric cells are used for the production of pluripotent ES cells, in particular human-like pluripotent ES cells. One disadvantage of this technique is that these chimeric cells may contain unknown non-human viruses and still contain the mitochondria of the animal species. Thus, there would be substantial risks of immune rejection if such cells were used in cell transplantation therapies.
In a fourth method, ES cells can be isolated from the primordial germ cells found in the genital ridges of post-implanted embryos.
As described above, all presently available methods depend on controversial sources—embryos (either created naturally or via cloning), fetal tissue and via the mixing of materials of multiple species. The controversy surrounding the sources for such cells, according to many leading scientists and public and private organizations including the NIH, has greatly compromised and slowed the study of such cells and their application.
There is thus a great demand for alternative methods of generating pluripotent cells.